R a d ar t e c h n i q u es u s i ng a r r ay a n t e n n as W u l f - D i e t er W i r th The Institution of Electrical Engineers Published by: The Institution of Electrical Engineers, London, United Kingdom © 2001: The Institution of Electrical Engineers This publication is copyright under the Berne Convention and the Universal Copyright Convention. All rights reserved. Apart from any fair dealing for the purposes of research or private study, or criticism or review, as permitted under the Copyright, Designs and Patents Act, 1988, this publication may be reproduced, stored or transmitted, in any forms or by any means, only with the prior permission in writing of the publishers, or in the case of reprographic reproduction in accordance with the terms of licences issued by the Copyright Licensing Agency. Inquiries concerning reproduction outside those terms should be sent to the publishers at the undermentioned address: The Institution of Electrical Engineers, Michael Faraday House, Six Hills Way, Stevenage, Herts. SG1 2AY, United Kingdom While the author and the publishers believe that the information and guidance given in this work is correct, all parties must rely upon their own skill and judgment when making use of it. Neither the author nor the publishers assume any liability to anyone for any loss or damage caused by any error or omission in the work, whether such error or omission is the result of negligence or any other cause. Any and all such liability is disclaimed. The moral right of the author to be identified as author of this work has been asserted by him/her in accordance with the Copyright, Designs and Patents Act 1988. British Library Cataloguing in Publication Data Radar techniques using array antennas. (Radar, sonar, navigation and avionics series; no. 10) 1. Radar 2. Antenna arrays I. Wirth, W. 621.3' 8483 ISBN 0 85296 798 5 Typeset by Newgen Imaging Systems, India Printed in England by Antony Rowe Ltd., Chippenham To Christiane Preface The intention of this book is to give the reader an introduction into the area of modern radar techniques based on active array antennas, either for use in development and research or as a decision and planning aid for the authorities within government or industry. In particular, many new possibilities are discussed which should find consideration for future radar systems. I have assumed the general basics of radar technology to be familiar to the reader; introductory literature is widely available. I have tried to present the techniques, procedures and concepts, which can mostly be derived from signal-theoretical views and which are described with approaches and equations, from an engineering point of view, with a minimum of mathematical treatment. As far as is possible, a descriptive and functional explanation is given for each procedure. Despite its already long history, radar technology will continue to receive attention for many years to come. The importance of radar results from its ability to provide reconnaissance, especially target detection, location and imaging, in all weather con- ditions and from short to long ranges. This ability results from the relatively small propagation attenuation of the electromagnetic waves with wavelengths in the cen- timetre to metre range. These wavelengths also permit sufficient location accuracies and resolutions with reasonable antenna dimensions. Radar works actively, illuminat- ing the scene to be observed with its own transmitter. From this results its ability to measure target range and its independence from daylight, in contrast to visual optics. One can thus expect future lively interesting and importance for radar technology. This statement is among other aspects confirmed by a well-established regular series of international radar conferences organised by respected and famous professional institutions such as IEE (UK), IEEE (USA), DGON/ITG (GE), SEE (F). The idea of writing this book was first developed by contributions and the sci- entific coordination of the training course 'Future radar systems' of the Carl Cranz society in Germany. Here in this book the material is presented in much more detail than is possible in a short course. Beyond this, the book is essentially based on experi- ences, implementations and new concepts which my colleagues and I have developed and collected in the course of more than 30 years of scientific work in the radar field at the Forschungsinstitut fur Funk und Mathematik (FFM, which translates into Research Institute for Radio Sensors and Mathematics) of the Forschungs- Gesellschaft fiir Angewandte Naturwissenschaften (FGAN, which translates into Research Association for Applied Natural Sciences). In the introduction a general representation of the substantial future requirements for radar systems is given. Afterwards, the future importance of multifunction sys- tems on the basis of electronically-steered array antennas is discussed. In the chapters 2, 3 and 4 there follows a short introduction into representation of signals, into the statistical signal theory and into the substantial characteristics, architectures and rela- tionships of array antennas. These array antennas may be of linear, planar, cylindrical or volume type. These chapters shall be used as a basis for the following chap- ters. In chapter 5 the receiver beamforming is discussed. Chapter 6 deals with the time-discrete signal sampling and derivation of the orthogonal signal components. Chapter 7 is dedicated to pulse compression with the use of polyphase codes, with special compression procedures for sidelobe reduction and an improvement of the range resolution. In chapter 8 we treat problems and possibilities for target detection from signal series, i.e. incoherent and coherent integration, Doppler filtering, adap- tive clutter suppression and the coherent processing of very long pulse series with a special test function. Chapter 9 presents sequential detection for radar application and its advantages, together with a solution for the problem of simultaneously testing multiple range bins. Chapters 10 to 15 are dedicated to concepts, procedures and applications of array signal processing. In chapters 10, 11 and 12 adaptive jammer suppression, correction of monopulse under jamming conditions and angular superresolution are discussed. In chapters 13 and 14 the additional possibilities and special problems of radar on flying platforms are presented, especially with respect to the detection of slowly moving tar- gets, that is space-time adaptive processing (STAP) procedures and synthetic aperture radar (SAR) with moving target indication (MTI) and target imaging. In chapters 15 the principles of inverse synthetic aperture radar (ISAR) are introduced as a way of generating radar images for detected moving targets. In chapter 16 fluctuation and spectral target characteristics, especially by jet engine modulation, are described. Both of these last chapters demonstrate the contributions to target classification which can be achieved with a multifunction radar. In chapter 17 a survey of the experimental phased array radar system ELRA which has been developed at FFM is given. In chapter 18 follows the description of a special system concept with omnidirectional or floodlight transmission and a multi- beam receiving system for radar operation with protection against antiradar missiles (ARMs). An additional application of this concept is the detection and classification of hovering helicopters. The corresponding experimental system OLPI following these concept ideas is described. Finally, in chapter 19 some remarks are made on system parameter relations and their choice for a multifunction radar system which is based on a phased-array antenna. I would like to thank all my colleagues at the department EL (electronics) of FGAN-FFM for my long, very fruitful and enjoyable time as department head. Especially I am grateful for many years of scientific cooperation, with numerous stimulating discussions, with W. Buhring, J. Ender, I. Groger, E. Hanle, R. Klemm, U. Nickel, W. Sander, K. V. Schlachta and H. Wilden. Their contributions to our work are also documented by the references at the end of each chapter. I would also like to thank Dr. Robin Mellors-Bourne, Diana Levy and Dr. Roland Harwood who, as the Book Publishing Department at the IEE, provided much support. The excellent co-operation that I received is very much appreciated. I also owe thanks to the editors of the IEE radar series, Professor E. D. R. Shearman and Professor P. Bradsell, and to the anonymous reviewers for their numerous corrections, creative comments and suggestions. Finally, I thank the director of the FFM Dr. Jiirgen Grosche and the head of the responsible department section within the German Ministry of Defence (BMVg) Hartmuth Wolff for the promotion of my work over many years in the exciting area of modern radar techniques and for the encouragement to write this book. Wulf-Dieter Wirth October 2000 Contents Preface ................................................................................ xv 1. Introduction ................................................................. 1 2. Signal Representation and Mathematical Tools ............................................................................ 5 2.1 Vectors, Matrices ........................................................... 5 2.2 Computing with Matrices ............................................... 6 2.2.1 Addition and Subtraction .............................. 6 2.2.2 Multiplication ................................................ 7 2.2.3 Identity Matrix .............................................. 8 2.2.4 Inverse Matrix .............................................. 9 2.2.5 Eigenvalue Decomposition ........................... 9 2.2.6 QR Decomposition ....................................... 10 2.3 Fourier Transform .......................................................... 10 2.3.1 Fast Fourier Transform, FFT ........................ 12 2.4 Filter in the Frequency and Time Domain ..................... 12 2.5 Correlation ..................................................................... 15 2.6 Wiener Khintchine Theorem .......................................... 16 2.7 References .................................................................... 17 3. Statistical Signal Theory ............................................ 19 3.1 The General Tasks of Signal Processing ...................... 19 3.2 Introduction to Basics of Statistics ................................. 20 3.2.1 Probabilities for Discrete Random Variables ...................................................... 21 This page has been reformatted by Knovel to provide easier navigation. vii viii Contents 3.2.2 Continuous Random Variables ..................... 23 3.2.3 Functions of Random Variables ................... 26 3.2.4 Statistical Averages ..................................... 30 3.2.5 Correlation ................................................... 32 3.2.6 Gaussian Density Function .......................... 32 3.2.7 Correlated Gaussian Variables .................... 33 3.2.8 Complex Gaussian Variables ....................... 34 3.3 Likelihood-ratio Test ...................................................... 35 3.4 Parameter Estimation .................................................... 40 3.4.1 Variance of the Estimate and Cramér-Rao Limit ........................................ 41 3.5 Estimation of a Signal .................................................... 45 3.5.1 Maximum-likelihood Estimation .................... 45 3.5.2 Signal Estimation with Least-mean- square Error ................................................. 46 3.5.3 Interference Suppression by the Inverse Covariance ...................................... 47 3.5.4 Improvement of Signal-to-noise-and- interference Ratio (SNIR) ............................. 49 3.6 Summary ....................................................................... 49 3.7 References .................................................................... 50 4. Array Antennas ........................................................... 51 4.1 Array Factor ................................................................... 52 4.2 Array Parameters .......................................................... 57 4.2.1 Half-power Beamwidth ................................. 58 4.2.2 Bandwidth Limitation with Phase Steering ....................................................... 58 4.2.3 Antenna Element Spacing Without Grating Lobes .............................................. 60 This page has been reformatted by Knovel to provide easier navigation. Contents ix 4.2.4 Gain of Regularly Spaced Planar Arrays with d = λ/2 ....................................... 61 4.2.5 Reduction of Sidelobes by Tapering ............ 61 4.3 Circular Array ................................................................. 64 4.4 Phase and Amplitude Errors ......................................... 65 4.5 Architectures of Passive and Active Array Antennas ........................................................................ 71 4.5.1 Comparison of Efficiency for Active and Passive Arrays ............................................. 75 4.5.2 Radar Equation for Active Arrays ................. 75 4.6 Concepts for an Extended Field of View ....................... 76 4.6.1 Volume Array for Complete Azimuth Coverage ..................................................... 77 4.7 Monitoring of Phased-array Antennas .......................... 79 4.7.1 Antenna Measurement ................................. 80 4.7.2 Transmit/Receive Module (TRM) Monitoring .................................................... 81 4.8 Appendix: Taylor and Bayliss Weighting ....................... 83 4.9 References .................................................................... 85 5. Beamforming ............................................................... 87 5.1 Single Receiving Beam ................................................. 89 5.1.1 RF Beamforming .......................................... 89 5.1.2 Subarrays and Partial Digital Beamforming ............................................... 89 5.1.3 Dynamic Range Requirements .................... 91 5.1.4 Subarray Configuration for Digital Sum and Difference Beamforming ....................... 92 5.1.5 Correction of Antenna Failures .................... 96 5.1.6 Digital Beamforming at Element Level ......... 97 5.2 Broadband Beamforming .............................................. 97 This page has been reformatted by Knovel to provide easier navigation. x Contents 5.3 Multiple Beams .............................................................. 101 5.3.1 RF Multiple Beamforming ............................. 102 5.3.2 RF Multiple Beamforming Using Subarrays .................................................... 103 5.3.3 IF Multiple Beamforming by a Resistive Network ....................................................... 104 5.3.4 Baseband Multiple Beamforming ................. 104 5.3.5 Time-multiplex Beamforming for Arbitrary Directions ...................................... 104 5.3.6 Digital Multiple Beamforming Using Subarrays .................................................... 104 5.3.7 Multiple Beam Cluster for Target Search ......................................................... 106 5.4 Deterministic Spatial Filtering ........................................ 107 5.5 References .................................................................... 109 6. Sampling and Digitisation of Signals ........................ 111 6.1 Analytical Signal ............................................................ 111 6.2 Sampling and Interpolation ............................................ 114 6.3 Extraction of the Components I and Q in Digital Format ............................................................................ 116 6.4 Third-order Intercept Point (TOI) and Dynamic Range ............................................................................ 121 6.5 References .................................................................... 123 7. Pulse Compression with Polyphase Codes ............. 125 7.1 Introduction .................................................................... 125 7.2 Requirements and Basic Structure for Pulse Compression .................................................................. 126 7.3 Binary Phase Codes ...................................................... 127 7.4 Polyphase Code as an Approximation of Linear Frequency Modulation ................................................... 128 This page has been reformatted by Knovel to provide easier navigation.